Page 105 - Electrical Equipment Handbook _ Troubleshooting and Maintenance

AC MACHINE FUNDAMENTALS

5.6 CHAPTER FIVE

P

e m
2
Since the mechanical frequency f n /60, the electrical frequency in hertz is related to
m m
the mechanical speed of the magnetic fields in revolutions per minute by
n P
m
f
e
120
Reversing the Direction of the Magnetic Field Rotation
The direction of the magnetic field’s rotation is reversed when the current in any two of
three coils is swapped. Therefore, it is possible to reverse the direction of rotation of an ac
motor by just switching any two of the three phases (Ref. 1).

THE INDUCED VOLTAGE IN AC MACHINES

Just as a rotating magnetic field can be produced by three-phase set of currents in a stator,
a three-phase set of voltages in the coils of a stator can be produced by a rotating magnetic
field.

The Induced Voltage in a Coil on a Two-Pole Stator
Figure 5.5 illustrates a stationary coil with a rotating magnetic field moving in its center.
The induced voltage in a wire is given by
e (v × B) •l
ind
where v velocity of wire relative to magnetic field
B magnetic flux density of field
l length of wire
This equation was derived for a wire moving within a stationary magnetic field. In ac
machines, the magnetic field is moving, and the wire is stationary.
Figure 5.6 illustrates the velocities and vector magnetic field from the point of view of
a moving wire and a stationary magnetic field. The voltages induced in the sides of the coil
are
1. Segment ab. The angle between v and B in segment bc is 180° , while the quantity
v B is in the direction of l, so

e (v × B)•l
ba
υBl sin (180° ) directed into page
The direction of e is given by the right-hand rule. By trigonometric identity, sin (180°
ba
) sin . So

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